Department of Earth Science and Engineering, Imperial College London
and Department of Earth Sciences, Natural History Museum
PhD Project 2016
Fingerprinting fertile porphyry magmas using apatite
Ore deposits are exceedingly scarce. Only one in every one thousand prospects that
are explored by companies is eventually developed into a mine, and the location of
such prospects has already consumed significant time and resources. Consequently,
understanding what controls the location of ore deposits, recognizing the potential
fertility of a particular belt of rocks before too much costly and invasive exploration
has taken place, and exploring efficiently within such domains are important for
reducing risk, energy use and environmental impact.
Porphyry-type ore deposits source much of the copper, molybdenum, gold and
silver utilized by humankind and are the largest and most complex geochemical
anomalies known in the Earth’s crust. They are related to magma generation in
subduction zones, the place where most of Earth’s continental crust is created. Most
of the time, the magmas produced are erupted or are emplaced at depth as barren
plutons. Rarely, they are transformed in such a way that they become capable of
generating abundant, metalliferous hydrous fluids that are released as the magmas
rise, cool and crystallize. At present, there is a limited understanding of what
triggers the change in magma fertility and controls the subsequent mineralizing
potential of exsolved solutions.
Our approach is to develop apatite (Ca5(PO4)3(F,Cl,OH)) as a probe of arc magma
evolution and of their exsolved fluids. In particular, we seek to identify processes
that “fertilize” arc magmas, transforming them from principally agents of
continental growth into porphyry and epithermal ore-generating systems. Four key
parameters are likely to be water content, oxidation state, halogen abundance
(ligands for metal transport), and sulphur (required to form ore minerals). Apatite
is extremely useful because it: 1) is a common accessory mineral in these rocks; 2)
tracks primary magmatic parameters such as aluminosity; 3) incorporates Ce, Eu
and Mn which vary according to magma oxidation state; 4) incorporates SO3, Cl and
F, monitoring melt S and halogens; and 5) records the Sr and Y content of melts at
the time of crystallization. Sr and Y track plagioclase and amphibole fractionation,
processes that are suppressed and enhanced, respectively, in water-rich melts,
leading to unusually high Sr/Y for a given silica content. Such “adakite-like”
signatures have been widely recognised in association with porphyry ore belts.
Apatite also forms from hydrothermal fluids released from crystallising intrusions.
This preserves a record of fluid halogen activities, and redox- and fluid salinitysensitive trace elements such as Mn, and hosts fluid inclusions representative of
mineralizing fluids.
In this project, we will establish crystallization history and apatite chemistry
in igneous rocks from mineralized (e.g. Miocene of Central Chile) and unprospective
(e.g. Neogene-Quaternary of the Southern Chilean Volcanic Zone) arc segments. In
addition, the CASE partner will provide a unique archive of apatite grains separated
from more than 25 variably mineralized porphyry systems worldwide. Methods
used will be: petrographic and fluid inclusion study (microscopy, SEM and
cathodoluminescence (CL) techniques, including novel, panchromatic hot-cathode
CL); microprobe (light elements such as F, Cl and S) and LA-ICPMS (for elements
down to ppb level in apatite and fluid inclusions) at the NHM. Determination of
some elements is non-routine but required analytical development has been
completed in a recently completed PhD project. Results will allow us to identify and
model key differences in magmatic evolution pathways in mineralized and barren
arcs/systems. Previously published LA-ICPMS studies of apatite-hosted fluid
inclusions are lacking but we have experience of this from a recently completed PhD
study. Processing of fluid inclusion ablation data is difficult for such minerals but
can be achieved using a host mineral correction, incorporated in our in-house
processing software ExLAM.
We are looking for a well-qualified and highly motivated Earth Sciences/Geology
graduate who wishes to carry out a cutting edge PhD in economic geology.
Excellence in geochemistry and mineralogy are essential; experience of
microanalytical techniques and statistical data evaluation are desirable.
Involvement with the Imperial Student Chapter of the Society of Economic
Geologists and outreach activities at the Natural History Museum will be
encouraged.
Closing date for applications: 15th January, 2016
For further details contact: j.wilkinson@nhm.ac.uk